Multiple Codes, help requested

[dgzieg]

Hi, long time lurker, first time poster. My '94 LSI started intermittently idling rough a couple of months ago. Seemed like the hotter the weather, the more likely I'd experience a rough idle. Since it was intermittent it was difficult to make a correlation. Anyway, finally started to get CEL indication. After a couple of weeks of the CEL lighting briefly (not even every day), I pulled the codes. I got 37 (Oxygen sensor #2, left side), 21 (Coolant Temperature Sensor), and 56 (EGR system). Is it likely that I've actually got three things wrong or is one of these likely to be causing the three faults? I've got no problem buying the necessary parts (maybe some problem installing them, I'm not as young as I used to be . If replacing the sensor(s) is the way to go, does anyone know the appropriate numbers to order by? How would you proceed? Thanks for any input, the combined expertise of this forum is amazing.